Conductivity of grains in nonlinear oxide ceramics and protective properties of varistors

Abstract

The current-voltage characteristics of various compositions of nonlinear varistor ceramics based on ZnO (ZOC) and SnO₂ (TOC) are studied in a wide range of electric current. On the basis of obtained data were determined the parameters that allow finding the coefficients k_prs and k_prl, which define the protection levels against the switching surge and the lightning surge for surge suppressors on the basis of ZOC and TOC varistors. The lower the value of these coefficients are, the significantly higher the efficiency of surge protection is. A relationship between k_prs, k_prl, and the effective linear resistivity ρ_lin limiting the current of intergranular barriers in varistor ceramics are established. It is shown that the values of k_prs and k_prl increase with increasing ρ_lin, which value is determined by the bulk resistance of the ceramic grains. The studied TOC compositions have 2-3 times larger values of ρ_lin and more high values of k_prs and k_prl than ZOC. ZOC-based varistors with lower k_prs and k_prl values provide more effective surge protection than TOC-based varistors.